Calculating Magnification | You can change the power of your Celestron |
| ing the eyepiece (ocular). To determine the magnification for your telescope, you |
| would simply divide the focal length of the telescope (1200mm) by the focal length |
| of the eyepiece that you are using. In equation format, the formula looks like this: |
| Focal Length of Telescope (mm) |
| |
| Focal Length of Eyepiece (mm) |
Determining Field of View
Let’s say, for example, that you are using a 20mm eyepiece. To determine the magnification, simply divide the focal length of your telescope (1200mm) by the focal length of the eyepiece (20mm). Dividing 1200 by 20 yields a magnification of 60 power.
Although the power is variable, each instrument has a limit to the highest useful magnification. The general rule is that 60 power can be used for every inch of aperture. For example, the
Determining the field of view is important if you want to get an idea of the size of the object you are observing. To calculate the actual field of view, divide the apparent field of the eyepiece (supplied by the eyepiece manufacturer) by the magnification. In equationformat,theformulalookslikethis:
ApparentFieldofEyepiece
Magnification
Using the example we started with above, we can determine the field of view using the same 20mm eyepiece. The 20mm eyepiece has an apparent field of view of 52°. Divide the 52° by the magnification, which is 60 power. This yields an actual field of .87°, or a little more than three quarters of a degree.
For terrestrial viewing, field size is often referred to as feet at a thousand yards. To convert this to feet at one thousand yards, multiply the actual field of .87° by 52.5. This produces a field width of 45.7 feet at one thousand yards.
The apparent field of each eyepiece that Celestron manufacturers is found in theCelestronAccessoryCatalog(#93685).
